1. Technical Field
The invention relates to a zoom lens suitable for, for example, a video camera and a digital still camera, and relates to an imaging apparatus including the zoom lens.
2. Description of the Related Art
In recent years, a rear focus type zoom lens has been generally used as a zoom lens for a video camera. The rear focus type zoom lens includes four lens groups having positive, negative, positive, and positive refractive powers in order from an object side, and the fourth lens group is used for focusing. There is an increasing demand for a video camera is not only compact and lightweight but also has a high performance, with an improvement in the image quality of high definition television monitors and with an increase in the screen size of high definition television monitors. Also, there is a demand for a video camera capable of capturing an image at a wide angle in a small space, such as in a room. Therefore, it is necessary to achieve a zoom lens having a large angle of view at a wide-angle end and a high zoom ratio.
There are some methods to achieve a size reduction of the four-group zoom lens having positive, negative, positive, and positive refractive powers, while maintaining a high performance. First, a movement of a second lens group in an optical axis direction may be reduced in order to increase a negative refractive power of the second lens group to ensure a predetermined zoom ratio. To this end, it is preferable that the second lens group include four or more lenses. In particular, when the second lens group consists of four lenses, it is preferable that the second lens group consists of, in order from the object side, a negative lens having a concave surface which has a strong refractive power and is directed to the image side, a negative lens having concave surfaces on both sides, and a cemented lens formed of a positive lens having convex surfaces on both sides and a negative lens (see JP 2001-91830 A and JP 2001-228395 A).
Also, the third lens group may be formed of a so-called telephoto type which includes two lenses having a positive refractive power and a negative refractive power in order from the object side, and the principal position of the third lens group may be moved to the object side to shorten a distance from the third lens group to an image formation surface. To this end, it is preferable that the third lens group include two or more lenses. In particular, when the third lens group consists of two lenses, it is preferable that the third lens group consists of, in order from the object side, a positive lens having a convex surface, which has a stronger refractive power and is directed to the object side, and a negative meniscus lens which has a concave surface, which has a stronger refractive power and is directed to the image side, (see JP Hei.4-43311 A (corresponding to U.S. Pat. No. 5,189,558), JP Hei.5-297275 A (corresponding to U.S. Pat. No. 5,396,367) and JP Hei.8-304700 A).
Further, the fourth lens group may be formed of a telephoto type, and the principal position of the fourth lens group may be moved to the object side to shorten a distance from the fourth lens group to the image formation surface. To this end, it is preferable that the fourth lens group include three or more lenses. In particular, when the fourth lens group consists of three lenses, it is preferable that the fourth lens group consists oft in order from the object side, a positive lens, and a cemented lens, which has a negative refractive power and is formed of a negative lens and a positive lens (see JP 2005-326516 A (corresponding to US 2005/0254138 A)).
However, although the second lens group of the zoom lens disclosed JP 2001-91830 A has the above-mentioned preferable structure, the fourth lens group consists of two lenses. Therefore, there is a limit to achieve a high-performance optical system. Also, S although the second lens group of the zoom lens disclosed in JP 2001-228395 A has the above-mentioned preferable structure, the third lens group consists of one lens. Therefore, it is difficult to shorten the overall length of the zoom lens, and it is difficult to achieve a high-performance optical system. Further, since a negative lens is disposed on the object side of the fourth lens group, a back focal length is long, and it is difficult to shorten the overall length of the zoom lens.
In the zoom lenses disclosed in JP Hei.4-43311 A (corresponding to U.S. Pat. No. 5,189,558), JP Hei.5-297275 A (corresponding to U.S. Pat. No. 5,396,367) and JP Hei.8-304700 A, the third lens group has the above-mentioned preferable structure. However, the second lens group consists of three lenses. Therefore, when the refractive power is increased to shorten the overall length of the zoom lens, the negative Petzval sum is increased, and a large image plane variation is caused by zooming. In addition, since the fourth lens group consists of one or two lenses and is not a telephoto type, it is difficult not only to achieve a high-performance optical system but also to shorten the overall length of the zoom lens.
Further, in the zoom lens disclosed in JP 2005-326516 A (corresponding to US 2005/0254138 A), the fourth lens group has the above-mentioned preferable structure. However, a good balance between the size reduction of the zoom lens and the improvement of the performance thereof is not achieved. Therefore, it is necessary to optimize the structure of the zoom lens. In particular, an air gap between two lens elements in the fourth lens group is large, and the overall length of the fourth lens group is long. Therefore, when considering the movement of the zoom lens during zooming and during focusing, a large space is required between the third lens group and the image formation surface.